Find a better way to deal with zoom levels that are unsuported by the current layer

[pR0Ps]

Auto-zoom (and regular zoom) can leave the user on an invalid zoom level for the selected layer and just show grey. Also, layers that don't support the current zoom level are marked as disabled in the layer switcher which is a bit confusing.

Best solution would be to zoom the tiles in if the user goes past the max zoom level provided by the tileset. Pixelated images are better than no images at all.

See #24